1. Field of the Invention
The present invention relates to a manufacturing process for producing Portland cement and white Portland cement.
2. Description of the Prior Art
The importance of cement in a modern economy cannot be overemphasized. Cements may be defined as adhesive substances capable of uniting fragments or masses of solid matter to a compact whole, and more specifically, the plastic materials employed to produce adhesion between construction materials. Cements include gypsum plaster, common lime, hydraulic lime, and natural and synthetic Portland cements. Of these, Portland cement is the most important as it is the basic ingredient of modern concrete.
Portland cement is a finely ground powder, usually gray in color, which when mixed with water, binds together other materials (such as sand, gravel, crushed stone) to form concrete, which is the most widely used construction material. Almost all of the cement used in construction today is xe2x80x9cPortlandxe2x80x9d or manufactured hydraulic cement as opposed to xe2x80x9cnaturalxe2x80x9d cement widely used a century ago.
The important cementations compounds in Portland cement have been identified as tricalcium silicate and beta dicalcium silicate. Dicalcium silicate has several crystalline phases, each having different properties. The beta and gamma phases are those of interest in the production of cement. The beta phase of dicalcium silicate is hydraulic and thus a cementations compound, but the gamma phase is not since it does not react to any appreciable extent with water. Tricalcium silicate contributes to the high early strength of cement because it hydrates more rapidly than dicalcium silicate, but both compounds ultimately set to the same strength. Other compounds sometimes present in Portland cement are tricalcium aluminate, 12-calcium-7-aluminate and tetracalcium aluminoferrate.
Processes for the manufacture of Portland cement are known in the art and have been described in detail, for example in the book xe2x80x9cThe Cement Plant Operations Handbookxe2x80x9d by Philip A. Alsop and James W. Post, Tradeship Publications Ltd., 1st ed., 1995. Generally, the conventional methods of making Portland cement use as raw materials calcium carbonate (usually in the form of limestone), silica (usually sand or quartzite), alumina (usually in the form of clay or shale) and ferric oxide (often in the form of iron ore). These materials are quarried or mined, crushed, pre-homogenized and then ground. The ground raw materials are blended and fired in a rotary kiln. The first chemical reaction that occurs is the calcination of the calcium carbonate to produce calcium oxide and carbon dioxide. Sometimes calcining is carried out in a reaction chamber, known as a precalciner, prior to the raw feed entering the kiln. The temperature in the kiln is controlled in the range of 1200xc2x0 C.-1500xc2x0 C., depending on the raw materials that are used. At these temperatures, ferric oxide and alumina melt so the feed has about a 20-30% liquid content. Raw feed mixtures with a high ratio of ferric oxide:alumina require only kiln temperatures on the low end of the temperature range, while those with a low ratio of ferric oxide:alumina require temperatures on the high end. In the kiln, calcium oxide combines with silica to form dicalcium silicate and tricalcium silicate. The presence of liquid promotes the mixing of the feed materials and allows the formation of dicalcium silicate and tricalcium silicate at a reasonable rate. The ferric oxide and alumina combine with the calcium oxide to form tetracalcium aluminoferrate, calcium aluminate and/or 12 calcium 7 aluminate, depending on the exact composition of the new material. Because liquid is formed during the reactions in the kiln, the resulting material is in the form of pellets known as clinker. On leaving the kiln the clinker is cooled, usually by forced air convection in a clinker cooler. The clinker is then blended with a small proportion of gypsum, or other regulative additive, and ground to produce finished Portland cement.
White Portland cement is produced by the same general processes as are used to produce grey Portland cement, except that iron containing compounds are removed from the feedstock, and contact of the feedstock or clinker with iron containing components of the cement plant equipment is minimized. Additionally, the clinker exiting the kiln is cooled more quickly to reduce the oxidation of any remaining iron in the clinker.
The above generally describes known processes for the production of Portland cement, which, together with other processes, are well known to persons skilled in the art of Portland cement production. What is not known in the art is the production of Portland cement from waste furnace slag by-product from the production of magnesium metal.
Magnesium furnace slag is a residue from the manufacture of magnesium metal. Magnesium furnace slag has previously been considered a worthless product and has been dumped in large landfills and allowed to accumulate. Recently, the build up of waste magnesium furnace slag has become a problem and could threaten the long term viability of some magnesium plants. As a result, there is a need to reduce the amount of magnesium furnace slag being dumped into the environment and to have commercial uses for such slag.
An aspect of the present invention is a process for producing white Portland cement from magnesium furnace slag by the steps of removing iron containing compounds from magnesium furnace slag, blending the magnesium furnace slag with calcium oxide (CaO) or a compound able to produce calcium oxide upon heating, and with silicon dioxide (SiO2) to form a feedstock, heating the feedstock to a temperature and for an amount of time sufficient to form clinker, cooling the clinker in a way that minimizes the oxidation of iron containing compounds in the clinker, reducing the particle size of the clinker to that of a powder, blending the clinker with a powdered regulating additive to form white Portland cement, and minimizing contamination of the slag, feedstock and clinker by iron containing compounds during the steps in the process.
Another aspect of the present invention is a process for producing Portland cement from magnesium furnace slag by the steps of blending magnesium furnace slag with calcium oxide (CaO) or a compound able to produce calcium oxide upon heating, and with silicon dioxide (SiO2) to form a feedstock, heating the feedstock to a temperature and for an amount of time sufficient to form clinker, cooling the clinker, reducing the particle size of the clinker to that of a powder, and blending the clinker with a powdered regulating additive to form the Portland cement.